This invention relates to processes and compositions for dyeing hair and particularly for dyeing hair on the human head. More particularly, it concerns the use of 5,6-dihydroxyindole-2-carboxylic acid (sometimes referred to herein as DHICA) in such processes and compositions.
Efforts have been made in the past to provide a hair dyeing process that mirrors the formation of hairs' natural pigment--melanin. These efforts relied in general upon treatment of hair with a recognized intermediate of the melanin pathway--5,6-dihydroxyindole--DHI.
While the melanin formation can be successfully achieved by this approach, the results have left much to be desired. Firstly, DHI is a highly unstable compound and this presents considerable difficulty in product formulation. Secondly, the colors obtained on hair with DHI tends to be achromatic and thus require separate treatments to attain a broader range of shades.
Indeed color modulation of dihydroxyindole dyed hair has been taught to require either a pretreatment of the hair with a color modifier such as a metal ion, or iodide or a reducing agent prior to dye application. Alternatively, an oxidative posttreatment can be effectively used to the hair. Moreover, precursors such as 5,6-dihydroxyindole require a complicated synthesis which makes the use of this material expensive and often precludes its commercial use. The present invention eliminates the aforesaid disadvantages.